The present invention relates to determining tape position error caused by tape skew and module misalignment and to repositioning the tape to minimize write element position error.
Magnetic tape is commonly used to store voice and data information due to its reliability, cost efficiency, and ease of use. Magnetic tape may be made more useful and cost-effective by increasing the areal density of information stored on the magnetic tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, the increase in the density of data tracks requires a narrowing of the width of the data tracks, a narrowing of the spacing between data tracks, or both. As the data tracks are more closely spaced, positioning of the tape with respect to the tape head becomes more critical to reduce the possibility of errors introduced while reading or writing.
Tape heads generally include read elements for reading data from the magnetic tape and write elements for writing data to the magnetic tape. Typically, read elements may be formed in a read module with one read element for each data track that is to be simultaneously read. Similarly, write elements are manufactured into a write module, with one write element for each data track to be simultaneously written . Thin film construction techniques are used to achieve the small geometries required to produce read elements and write elements capable of accessing densely packed data tracks. Using thin film technology, spacing between read elements or write elements within a module can be preciously maintained. To permit read-after-write operation on tape moving in either tape direction over the tape head, a typical tape head may include a sandwich of one write module between two read modules.
In order to increase the accuracy of positioning the tape head relative to the tape, servo stripes or tracks may be used to provide a reference point . One or more servo stripes may be used depending upon the number of data tracks which are placed upon the tape. Servo read elements in the read modules sense tracking patterns on the servo stripe and produce signals which are received by a control system. The control system moves the head to keep the servo signals at nominal magnitude. The nominal signal occurs when the servo read element is located in a certain position relative to the servo stripe. The servo stripe may also contain features, such as synchronization fields, which indicate the beginning of a servo frame at the same location in each servo stripe across the width of the tape.
Several complications can arise that produce an error in the tape position relative to the head. First, the tape may be skewed at a skew angle relative to the tape head. Tape skew causes several difficulties. First, information written simultaneously in different tracks across the width of the magnetic tape crosses read elements and write elements at different times if the tape is skewed. Skew buffers may be required to compensate for time differences. A second difficulty results because the servo read elements are not vertically aligned with the write elements. Centering one or more servo stripes across servo read elements in a single read module will result in the data tracks not being aligned over the top of write elements due to the skew angle.
Another source of tape position error arises from the manufacture of the tape head. Within each read and write module, read and write elements can be positioned precisely relative to one another due to the accuracies permitted by thin film manufacturing processes. However, when the individual modules are assembled into the tape head, an offset may develop between modules. Such offsets introduce tape position errors. For example, an offset between a write module and a read module containing a servo read element produces a corresponding offset in the data tracks written by the write element when the tape is positioned across the read head using signals sensed from the servo stripe by the servo read element.
What is needed is a system and method for measuring tape position errors due to tape skew and module offset. The tape position error information should then be used to increase the accuracy of positioning the tape head relative to the magnetic tape.
It is a primary object of the present invention to detect and reduce tape position error in a tape deck.
Another object of the present invention is to provide for determining the skew of magnetic tape relative to servo readers.
Still another object of the present invention is to provide for positioning write elements to compensate for tape skew.
Yet another object of the present invention is to provide for determining offset between modules in a multiple module tape head.
A further object of the present invention is to provide for positioning write elements to compensate for offset between modules in a multiple module tape head.
In carrying out the above objects and other objects and features of the present invention, a system is provided for measuring position error of a magnetic tape. The tape has data tracks and servo stripes across the tape width. Each servo stripe has a periodically spaced sequence of synchronization fields and tracking patterns aligned with similar patterns in other servo stripes across the width of the tape. The system includes a multiple module tape head with at least one read module and at least one write module. A read module includes at least two servo read elements for detecting the synchronization fields. The system also includes a head control which can detect the presence of a synchronization field passing a first servo element and the presence of a corresponding synchronization field passing a second servo read element. The elapsed time between the detected synchronization fields is determined. The tape skew is found based on the determined elapsed time, the distance between the servo read elements, and the tape speed.
In an embodiment of the present invention, a second read module includes a servo read element aligned with a servo read element in the first read module such that both servo read elements read the same particular servo stripe. The head control detects a tracking pattern on the servo stripe passing the first read module servo read element and determines a first offset as the tape offset relative to the first read module servo read element. A second tracking pattern is detected from the servo stripe passing the second read module servo read element and a second offset is determined as the tape offset relative to the second read module servo read element. The offset of the second read module relative to the first read module is determined based on the skew, the first offset, and the second offset. In a refinement, the head controller estimates the offset of each write module relative to the first read module based on the determined offset between the first read module and the second read module and the distance between the first read module and the write module.
In another embodiment of the present invention, the write module includes at least one servo read element aligned with a servo read element in a read module such that both read elements read a particular servo stripe. The head control logic detects a first tracking pattern on the servo stripe passing the read module servo read element and determines a first offset as the tape offset relative to the read module servo read element. A second tracking pattern is detected on the servo stripe passing the write module servo read element and a second offset is determined as the tape offset relative to the write module servo read element. The offset of the write module relative to the read module is determined based on the skew, the first offset, and the second offset.
A tape deck for accessing a magnetic tape is also provided. The tape deck includes means for positioning the tape head across the width of the magnetic tape in response to a positioning signal. The head control generates the positioning signal based on the determined skew. In various embodiments, the positioning signal is also based on the offset between read modules or on the offset between one or more write modules and a read module.
A method for determining position error of a magnetic tape is also provided. The method includes detecting the presence of a synchronization field passing a first servo read element in a read module. The presence of a corresponding synchronization field passing a second servo read element in the read module is also detected. An elapsed time between the detected synchronization fields is determined. The tape skew is found based on the determined elapsed time and the tape speed.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.